Rusts are plant diseases caused by fungal pathogens of the order Pucciniales. Rusts can affect a variety of plants, including monocotyledons and dicotyledons, as well as various plant organs, including leaves, stems, fruits and seeds. Rust is typically observed as colored powdery pustules composed off tiny spores that form on the lower plant organ surfaces. Common rust-causing fungal species include Gymnosporangium juniperi-virginianae (Cedar-apple rust) which attacks apple and pear and hawthorn; Cronartium ribicola (White pine blister rust); which attacks white pines and currants; Hemileia vastatrix (Coffee rust) which attacks coffee plant; Puccinia graminis (wheat stem rust) which attacks Kentucky bluegrass, barley, and wheat; Puccinia coronata (Crown Rust of Oats and Ryegrass) which attacks oats; Phakopsora meibomiae and P. pachyrhizi (soybean rust) which attacks soybean and various legumes; Uromyces phaseoli (Bean rust) which attacks bean; Puccinia hemerocallidis (Daylily rust) which attacks Daylily; Puccinia persistens subsp. triticina causes (wheat rust in grains, also known as ‘brown or red rust’); P. sriiformis (yellow or stripe rust); Uromyces appendeculatus which attacks bean.
Wheat stem rust is caused by the fungus Puccinia graminis and is a significant disease affecting cereal crops, particularly wheat (Triticum spp.) An epidemic of stem rust on wheat caused by race Ug99 is currently spreading across Africa, the Middle East, and Asia, and threatening large numbers of people who are dependent on wheat for sustenance. The rust fungus attacks the parts of the plant which are above ground. Spores that land on green wheat plants form a pustule that invades the outer layers of the stalk. Where infection has occurred on the stem or leaf, elliptical blisters or pustules called uredia develop. Infected plants produce fewer tillers and set fewer seed.
Soybean rust is a disease that primarily affects soybeans and other legumes. It is caused by two species of fungi, Phakopsora pachyrhizi and Phakopsora meibomiae. Soybean rust has been reported in Asia, Australia, Africa, South America and North America. Soybean rust is spread by wind-borne spores, which are released in cycles of seven days to two weeks. Yield losses can be severe with this disease and losses of 10-80% have been reported.
Multi-pronged approaches are desirable to address the spread of fungal infection. A variety of preventative methods may be employed. For example, rust diseases are correlated to relatively high moisture. Accordingly, avoidance of overhead watering at night, using drip irrigation, reducing crop density, and the use of fans to circulate air flow may serve to lower the relative moisture and decrease the severity of rust infection.
Other strategies may include reducing the area of the plant that the pathogen destroys, or slowing down the spread of the fungus. Fungus-resistant plants may be used to interrupt the disease cycle because many rusts are host-specific. This approach has proven very successful in the past for control of wheat stem rust; however, Ug99 is currently virulent against most wheat varieties. Currently, there are no commercial soybean varieties with resistance to soybean rust. Accordingly, soybean rust is managed with fungicides.
In large agricultural operations, conventional synthetic fungicides can be used to control fungal pathogens. De-methylation inhibitors (DMI) such tebuconazole (Folicur™, Bayer) and propioconazole (Tilt™) may be effective for the control of wheat stem rust, while tetraconazole (Domark™, Valent) be effective for the control of soybean rust. Quinone Outside Inhibitors (QoI), such as pyraclostrobin (Headline™, BASF), may be effective for the control of soybean rust, while azoxystrobin (Quadris™) may be effective for the control of wheat stem rust.
Conventional fungicides are generally applied by air to the foliage as contact between the pathogen with the fungicide is required for efficacy. This process can expensive and fungicide application is often reserved for seasons when foliar diseases are severe. Second, conventional fungicides typically target specific sites of important pathogen proteins. Accordingly, strains may develop resistance to the fungicide after repeat applications. Third, the efficacy of conventional chemical fungicides is not always satisfactory for some of the diseases, such as Fusarium Head Blight (Scab). Finally, conventional fungicides are generally not acceptable for use in organic farming.
As an alternative approach to conventional chemical fungicides, oil-in-water emulsions comprising paraffinic oils (paraffinic oil-in-water emulsions) and paraffinic spray oils have been used in turfgrass management practices for controlling turfgrass pests (see, for example, Canadian Patent Application 2,472,806 and Canadian Patent Application 2,507,482). In addition, oil-in-water formulations comprising paraffinic oils and a pigment for controlling turfgrass pests have been reported (see, for example, WO 2009/155693). For example, Petro-Canada produces CIVITAS™, is a paraffinic oil-in-water emulsion that functions as a broad spectrum fungicide and insecticide for use on golf course turf and landscape ornamentals, used for example to control powdery mildew, adelgids and webworms on landscape ornamentals (US EPA REG. NO. 69526-13). Product labeling indicates that CIVITAS™ may be applied as part of an alternating spray program or in tank mixes with other turf and ornamental protection products; and that CIVITAS™ may be used as a preventative treatment with curative properties for the control of many important diseases on turf, including fairways and roughs.